1. Field of the Invention
The present invention relates, in general, to a method for the preparation of difluoromethane and, more particularly, to a method for reacting methylene chloride with hydrogen fluoride in liquid phase, applicable to industrial scale and superior in conversion rate of the materials and in production yield.
2. Description of the Invention
As compounds of chlorofluoro carbon (hereinafter referred to as "CFC") system which have extensively used for foaming agents, detergents, aerosol spraying agents, refrigerants and the like, are proved to be a main factor destructurizing the ozone layer of the stratosphere, there is increasingly demanded a substitute for CFC which less or little affects the ozone layer. In an effort to prevent the destructure of the ozone layer, hydrochlorofluoro carbon (hereinafter referred to as "HCFC") was developed. HCFC has an ozone depleting potential (hereinafter referred to as "ODP") of 0.02 to 0.1 which is somewhat lower than that of CFC. However, even though it is low in depleting ozone, HCFC still destructurizes the ozone layer. Owing to this, HCFC is destined to be prohibited from production and use in a few years, in accordance with protocol agreed internationally.
Accordingly, intensive research and study have been directed to development of substitutes that destructurize, by no means, the ozone layer, that is, have an ODP of zero as well as function equally to CFC. As a result of the research and study, a hydrofluoro carbon (hereinafter referred to as "HFC") system was developed. In the HFC system, there are known a variety of compounds, such as HFC-32, HFC-125 and HFC-134a. In the future, it is believed that HFC-32 is rapidly substituted for chlorodifluoro methane (hereinafter referred to as "HCFC-22") used at present, in a form of mixture refrigerant of HFC-32/HFC-134a (25/75 wt %) or HFC-32/HFC-125/HFC-134a (30/10/60 wt%), or in a form of azeotropic refrigerant of HFC-32/HFC-125 (60/40 wt%). The abbreviations HFC-134a and HFC-125 as used herein denote tetrafluoroethane (CF.sub.3 CH.sub.2 F) and pentafluoroethane (CF.sub.3 CHF.sub.2), respectively.
HFC-32 may be prepared by two methods: gas phase method and liquid phase method. In the gas phase method, methylene chloride (CH.sub.2 Cl.sub.2) and hydrogen fluoride (HF) are preheated and reacted with each other in gas phase, in the presence of a metal catalyst, such as Al or Cr-based catalyst well known to the art. On the other hand, the gas phase method, as implied by its name, is performed by reacting methylene chloride with hydrogen fluoride in liquid phase, in the presence of catalyst, but has not been adopted commercially, thus far.
The reaction processes for HFC-32 proceed sequentially and are as follows: ##STR1## In the art, CH.sub.2 ClF and CH.sub.2 F.sub.2 are typically called HCFC-31 and HFC-32, respectively. As apparent from the above reaction formula, HCFC-31 is reclaimed in a reactor such that it reacts with HF, again, to produce HFC-32.
European Patent No. 0128510 suggests a preparation method for HFC-32 wherein methylene chloride and hydrogen fluoride are preheated and then, gaseous methylene chloride is reacted with gaseous hydrogen fluoride in the presence of a catalyst selected from a chrome-based compound, such as Cr.sub.2 O.sub.3 and CrF.sub.3, an aluminum-based compound, such as Al.sub.2 O.sub.3,AlCl.sub.3 and AlF.sub.3, and the combinations thereof. In the case that a combination of the aluminum-based and the chrome-based compounds is employed as a catalyst, the aluminum-based compound is added in an amount of 0.1 to 50 parts by weight based on unit part of the chrome-based compound. The gas phase reaction of this European patent is carried out at a temperature of 200.degree. to 450.degree. C. under the atmosphere with the mole ratio of hydrogen fluoride to methylene chloride ranging from 1 to 20.
Japanese Patent Publication No. Sho. 58-100464 teaches that HFC-32 is prepared by vaporizing methylene chloride and hydrogen fluoride with heat and subjecting the resulting gaseous methylene chloride and gaseous hydrogen fluoride to gas phase reaction at a reaction temperature of 200.degree. to 500.degree. C. under atmosphere or pressure in the presence of a chrome catalyst, such as CrF.sub.3, CrCl.sub.3 and Cr.sub.2 O.sub.3, with the mole ratio of gaseous methylene chloride to gaseous hydrogen chloride ranging from 1 to 20.
Supra patents describe preparation of HFC-32 with methylene chloride and hydrogen fluoride in the presence of a chrome-based or aluminum-based catalyst, requiring the catalyst to be molded into a pellet type with a dimension of 4 mm.PHI..times.4 mmH or 4 mm.PHI..times.6 mmH wherein .PHI. and H represent diameter and height, respectively. However, the molding of the catalyst is troublesome. In addition, the gas phase reaction is relatively complicated because it needs a preheater and a mass flow controller in order to vaporize liquid materials and to provide the gaseous materials in a constant rate. Further, since the gas phase reaction is executed at high temperatures, it is difficult to control the reaction temperature as compared with the liquid phase reaction. Furthermore, the high reaction temperature of the gas phase method not only makes reaction vessel corroded but also expedites ageing of the catalyst. Particularly, the methods suggested in supra patents exhibit low conversion rate of material (for example, conversion rate of methylene chloride is in a range of 76 to 85% and conversion rate of hydrogen fluoride 18 to 34% in the above-mentioned European patent, and conversion rate of methylene chloride 70 to 84% and conversion rate of hydrogen fluoride 25 to 31% in the Japanese patent mentioned), so that various economical countermeasures for the unreacted materials, such as separation of methylene chloride and hydrogen fluoride, recovery, purification and recycle, should be taken.
European Patent No. 0508660 discloses that HFC-32 is prepared by replacing the chlorine of HCFC-22 with hydrogen gas at a reaction temperature 135.degree. to 140.degree. C. in the presence of a catalyst in which a catalytically active material, such as palladium (Pd), platinum (Pt), nickel (Ni) and protoactinium (Pa), is incorporated into a carrier of active carbon in an amount of 0.5 to 20% by weight.
European Patent No. 0508631 employs a complex metal hydride catalyst, such as lithium aluminum hydride (LiAlH.sub.4) and sodium brome hydride (NaBH.sub.4). In this patent, the chlorine atom of HCFC-22 is substituted by hydrogen atom at a temperature of 20.degree. to 71.degree. C. in the presence of the complex metal hydride catalyst, to give HFC-32.
However, the two just mentioned patents are disadvantageous in that HCFC-22, primary converted from chloroform (CHCl.sub.3), is used as a starting material. In addition, the conversion rate of HCFC-22 in the conventional methods is low, for example, on the order of 0.36 to 84.1%. Moreover, there is a serious problem of side reaction that byproducts, such as methane (CH.sub.4), trifluoromethane (CF.sub.3 H), monochloromethane (CH.sub.3 Cl), ethane (CH.sub.3 CH.sub.3), difluorodichloromethane (CF.sub.2 Cl.sub.2) and trifluoromonochloromethane (CF.sub.3 Cl), are produced along with the object compound. Nowhere in the two supra patents is mentioned separation and purification of the by-products.
The above-described gas phase reaction methods for HFC-32 in which HF and CH.sub.2 Cl.sub.2 are reacted at a temperature 200.degree. to 500.degree. C. in the presence of metallic catalyst (Al, Cr, Pa, Pt, Ni, etc.) with the mole ratio of HF to CH.sub.2 Cl.sub.2 ranging from l to 20 and preferable 5 to 10 have a significant problem that the conversion rate of material is extremely low, for example 15-35% for HF and 70-85% for CH.sub.2 Cl.sub.2. In turn, low conversion rate of HF and CH.sub.2 Cl.sub.2 causes other problems. For example, it is difficult to recover the materials. In addition, the remaining materials along with the product and by-products form azeotropes, from which the product are hard to separate and purify. Consequently, the production yield is lowered.
The high reaction temperature in the gas phase reaction, 200.degree. to 500.degree. C. includes possible troubles that the reaction vessels might be corroded and by-products may be produced abundantly.
U.S. Pat. Nos. 2,749,374 and 2,749,375 introduce a liquid phase reaction for the preparation of HFC-32 with a catalyst of antimony halide. In the Examples of the patents, SbF.sub.3 provided with Cl.sub.2 or a combination of SbF.sub.3 and SbCl.sub.5 is utilized as the catalyst. As for reaction conditions, 2 to 3 moles of hydrofluoride per mole of methylene are provided at a temperature of 110.degree. to 175.degree. C. under a pressure of 400 Lb/cm.sup.2.G. The catalyst is present in an amount of 0.2 to 2 moles per mole of methylene chloride, and Cl.sub.2 is added in such a way to make the concentration of Sb.sup.5+ at least 5%, with the aim of regenerating the catalyst.
However, this liquid phase reaction process has some problems. First, the catalyst is high in concentration (Sb:CH.sub.2 Cl.sub.2 =0.2-2:1) and rendered to be tar by the high reaction temperature. The tar catalyst may cause side-reaction. SbF.sub.3, the catalyst, is difficulties for its preparation and provision. In addition, the reaction rate upon SbF.sub.3 is slower than upon SbCl.sub.5. Further, the conversion rates of the materials are low: 83-89% for methylene chloride; 70% for hydrochloride. What is still worse, the catalyst is very expensive and thus, not suitable for commercial production. It is believed that the prior techniques described in the supra U.S. patents are difficult to apply for industrialization, in consideration of the use of water cooled down 8.degree. C. in a compressor, the high reaction temperature, and the batch system conducted in a laboratory scale.